This invention relates to a method of controlling the temperature of steel strip in continuous heating equipment and, more particularly, to a method of controlling the temperature of steel strip in continuous heating equipment such as a continuous annealing line.
Specifically, the invention relates to a method of controlling the temperature of steel strip in continuous heating equipment in which strips of different thickness, but with approximately the same temperature, and welded together at the entry end of the equipment, are continuously transported therethrough at a given speed, and heated to a desired temperature at the exit end thereof irrespective of the strip thickness.
Generally, continuous heating furnaces are used for continuously annealing steel strip. Specific heating patterns are established to impart desired formabilities to the strip material. Each heating pattern has a desired ultimate temperature to which the strip should be heated or with which the strip should leave the exit end of the continuous heating furnace irrespective of strip thickness.
Such heating furnaces can be broadly classified into those which are heated electrically (either by direct excitation or by induction heating) and those heated by burning fuel gas. The gas-fired furnaces can be subclassified into the radiant-tube type and the direct-fired non-oxidizing atmosphere type.
Considering energy efficiency, running cost, initial investment and other factors, the gas-fired furnaces are much more advantageous than the electrically heated ones.
When continuously heat-treating strips of different thickness, it is a common practice to weld them together on a welder before feeding them to the heating furnace. Even when the strip thickness changes like this, the strip temperature at the exit end of the heating furnace should be maintained unchanged.
Conventionally, the exit-end temperature of such differential-thickness strip has been controlled by adjusting the temperature of the continuous heating furnace (i.e., the temperature of the furnace atmosphere).
For example, in a radiant-tube furnace with a regular heating rate of 15.degree. C. per second, the exit-end temperature of the differential-thickness strip can be satisfactorily controlled by said furnace temperature adjustment. Because the furnace temperature need not be changed extensively, only a short length of the strip fails to reach the desired exit-end temperature, creating no yield problem.
Recently, however, methods have been proposed to heat the strip at a rapid rate such as 100.degree. C. per second or above in a continuous-annealing process, the object of which is to obtain cold-rolled strip with excellent formability. In such high-speed operations, the furnace temperature cannot be adjusted as quickly as required, so that the incorrectly heated portion in the strip increases and a yield problem arises.
This decreased yield problem will be explained with a concrete example. Let it be assumed that strip having a thickness of 0.6 mm is heated within a given range (e.g. from approximately 700.degree. C.) in a heating furnace of a given length (e.g., 20 m), while being transported at a fixed speed of 400 m per minute. Within the above heating range, the strip is heated at a rate of 100.degree. C. per second to attain a constant temperature of 700.degree. C. at the exit end of the furnace. When the strip thickness changes from 0.6 mm to b 0.4 mm, the above operating conditions cannot be maintained unless the preset furnace temperature is changed by 100.degree. C. With such a temperature adjustment due to the changes in thickness of the strip, approximately 20 m of the strip will be heated to a temperature which deviates from the desired temperature. The tail end of one strip and the head end of the next strip welded thereto will have an off-gauge portion of approximately 10 m each on both sides of the weld. The above-mentioned incorrectly heated length should ideally correspond to the total length of the off-gauge portions on both sides of the weld. To confine the incorrectly heated length within this off-gauge length, the 100.degree. C. adjustment in the furnace temperature should be accomplished in 3 seconds. But such a quick change cannot be achieved using existing techniques and equipment.
For example, a continuous heating furnace with an ordinary furnace temperature control system will require 5 to 10 minutes to complete the 100.degree. C. adjustment in the furnace temperature. Consequently, the length of the strip which fails to reach the desired temperature is 2000 m to 4000 m, which means that a considerable length of strip having an acceptable thickness must be discarded as scrap due to having been improperly heated.
Even when the welded strip does not contain any off-gauge portion, the incorrectly heated part, of course, must be scrapped. The off-gauge length depends on the accuracy of the automatic gauge control system of the cold tandem mill.